A conventionally-known turbocharger rotates a turbine wheel by utilizing energy of exhaust gas taken in from an engine and rotates a compressor wheel disposed coaxially with the turbine wheel to supply an intake manifold with pressurized air, thereby improving an output. If such a turbocharger is to be mounted to a vehicle, efficiency of the turbocharger may affect fuel consumption of an engine. Thus, to improve fuel efficiency in particular, weight reduction, cost reduction, facilitated fabrication, and heat-capacity reduction are required.
Recently, an air-fuel ratio of an engine of an automobile is brought closer to a stoichiometry to reduce fuel consumption, and thus an exhaust-gas temperature tends to increase. Thus, for a common casted turbine housing, expensive materials such as austenite stainless cast steel, which has a high heat-resistance property and a high abrasion-resistance property, for instance, are used to resist high-temperature exhaust gas. Further, a casted turbine housing has a rough surface roughness on an inner wall surface of a housing. Thus, machining is required to make up for the insufficient accuracy of casted parts to prevent an increase in loss of exhaust gas flowing through an exhaust-gas passage in the housing and an accompanying decrease in efficiency of a turbocharger. Accordingly, such a casted turbine housing may increase material cost and production cost, for instance, and discourage cost reduction of a turbocharger. Thus, a sheet-metal turbine housing tends to be employed instead of a casted turbine housing.
As an example of a sheet-metal turbine housing, for instance, Patent Document 1 discloses a turbine housing including a scroll part with an exhaust gas passage of a scroll shape formed inside, the scroll part being formed by butting halved sheet-metal members each having a plate shape or a bowl shape and welding the sheet-metal members in a circumferential direction (see FIG. 3 of Patent Document 1). Further, Patent Document 2 also discloses a turbine housing including a scroll part formed by butting two right-and-left halved sheet-metal members and welding the sheet-metal members in a circumferential direction (see FIG. 2 of Patent Document 2). Further, Patent Document 3 discloses a turbine housing including a sheet-metal scroll member with an exhaust-gas passage of a scroll shape formed inside, and a sheet-metal cover member surrounding the outer periphery of the scroll member (see FIG. 4 of Patent Document 3).
Further, Patent Document 4 discloses a casted turbine housing with a sheet-metal inner peripheral surface formed inside during casting of the turbine housing (see FIGS. 1 and 2). Patent Document 5 discloses a turbine housing including a casted turbine casing with a ceramic scroll part formed inside during casting of the turbine casing (see FIG. 1).